January 2013

Decoding Loading: In vivo data recast shoe–OA discussion

In the moment: Footwear

A German study of individuals with instrumented knee implants suggests the actual effects of footwear on loading in patients with knee osteoarthritis (OA) may be much more subtle than those reported in previous studies using indirect measures of load.

In six individuals with instrumented knee implants, researchers from Charite Medical University Berlin found that the contact forces on the medial knee compartment experienced while walking in four different types of footwear did not differ significantly from barefoot walking. The findings were e-published in December by the Journal of Biomechanics.

The study participants were about 1.5 years postarthroplasty for end-stage knee OA and all were pain free. They walked on a treadmill at a constant 4 km/h while barefoot and while wearing, in random order, four types of shoes: basic (a low-cost running shoe), high level (advanced, high-cost running shoe), unstable (casual shoe with soft rounded sole), and dress (classic dress shoe with 2.5-cm heel).

Peak medial contact force in early stance (first peak) was 3% to 5% higher in all shoes than during barefoot walking; no increases were statistically significant. Peak medial contact force in late stance (second peak) increased by 8% with the dress shoe, decreased by 2% to 5% with the high-level and unstable shoes, and did not change with the basic shoe; again, however, no changes were statistically significant.

“The study showed that footwear had only a relatively moderate effect on joint loading,” said Ines Kuztner, Dr.rer.medic, a postdoctoral fellow at the university’s Julius Wolff Institute for Biomechanics and Musculoskel­etal Regeneration and first author of the study. “It also tends to support the belief that high-heeled shoes increase joint loading. However, a general conclusion is limited due to the fact that only six subjects participated in this study.”

The findings also raise questions about the popular trend in research of using peak external knee adduction moment (KAM), an indirect measure of knee loading calculated from marker-based kinematic data using inverse dynamics, as a surrogate for peak medial contact force. A number of studies have found that KAM is significantly higher when participants wear some types of footwear than when they are barefoot.

The Berlin group’s footwear study did not measure kinematics or calculate external moments. But a second study from the same group, presented last summer at the annual congress of the European Society of Biomechanics, compared peak medial contact force and peak KAM in eight individuals with instrumented knee implants and found a strong correlation only for early stance.

That study also found that, even when the correlation was strong, a change in KAM did not necessarily correspond to the same percentage change in peak medial contact force.

Darryl D’Lima, MD, PhD, who has also done studies on patients with instrumented implants at the Scripps Clinic in San Diego, CA, said his experience regarding external moments has been similar.

“The use of external knee adduction moment as a surrogate for medial knee loading has mixed value,” D’Lima said. “We have found some general statistical correlation between the peak knee adduction moment and the peak medial forces, but the correlation over the entire gait cycle and across multiple subjects is often not as robust as I would like.”

D’Lima also noted, however, that differences in study populations must be considered.

“Instrumented implants are the present state-of-the-art for measuring knee forces and there­­fore can be useful in predicting knee forces in patients with osteoarthritis,” he said. “However, this should be tempered by the fact that patients with these implants are relatively pain-free, do not have the arthritic deformities and ligament pathology that are common in osteoarthritis, and usually lack an anterior cruciate ligament.”

Using external knee adduction moments is not a “popular trend” it is a scientifically proven calculation of forces across the joint segment. Both researchers indicated that these calculated moments are correlated to the internal measured loads. Be careful about drawing conclusions from a study with only a few subjects at one point in time when there are many more published studies of large numbers of subjects over many months with data such as pain levels, bone mineral density and knee adduction angular impulse showing changes over time. Finally they are post-surgical patients presumably in end stage OA (K-L 4) while most studies using knee adduction moments are Kellgren-Lawrence stage 2 0r 3. Simple going in surgically has mechanically altered the joint so it is sort of like comparing apples to oranges.